1. Field of the Invention
The present invention relates to an image display device, and specifically, though not exclusively, relates to image modulation using liquid crystal modulators.
2. Description of the Related Art
Hitherto, conventional liquid crystal modulators, such as two-dimensional pixel optical switches, that can serve as an image modulation device used in a projection-type image display device, and liquid crystal projectors using such liquid crystal modulators. Of the liquid crystal modulators used in a liquid crystal projector, there are so-called TN (Twisted Nematic) liquid crystal modulators for example. The TN liquid crystal modulators are a configuration where nematic liquid crystals, which have positive dielectric anisotropy, is sealed between a first transparent substrate having a transparent electrode and a second transparent substrate having transparent electrodes, wiring, and switching devices which form pixels. The major axis of liquid crystal molecules can be twisted 90 degrees continuously between the two transparent substrates.
Also, other than such transmissive liquid crystal modulators, there are reflective liquid crystal modulators which have a reflective mirror inside one of the substrates as a two-dimensional pixel optical switch.
Liquid crystal modulators use an ECB (Electrically Controlled Birefringence) effect, and are used to control the polarization state and form an image. Of these, the liquid crystal modulators generally used are those in TN mode operation, where the nematic liquid crystals of which dielectric anisotropy is positive are homogeneously aligned spirally, and where optical switching is performed with liquid crystal birefringence.
In the event that TN mode is used and modulation is controlled with the ECB effect, in a state where there is no voltage applied to a liquid crystal layer, the liquid crystal molecules which differ in refractive index in the diameter direction (minor axis direction) and the liquid crystal molecule major axis can be arranged in an approximately 90 degree twisted spiral on a plane which is roughly perpendicular to the thickness direction of the liquid crystal layer. Therefore the liquid crystal layer has birefringence as to a predetermined direction of the plane, applies retardation (optical path difference between two light fluxes with differing polarization direction) as to a light wave which transits through the liquid crystal layer, and effects change to the polarization of the light wave.
With a general liquid crystal modulator design, incident light is changed to a linearly polarized state with light wave polarization in a predetermined direction, by a polarization control device such as a polarizer. Then the obtained light wave is cast into the liquid crystal layer, and when the linearly polarized light which oscillates in this predetermined direction transits through the liquid crystal layer, only a half-wavelength of retardation is applied to the incident light wavelength (e.g., a center-wavelength in a given light wavelength band).
The light having transited through the liquid crystal layer has the oscillating direction changed to the direction at right angles with (perpendicular to) the oscillating direction of the linear polarization before the light being cast in, and the light is emitted.
After this, the polarization state is selected by the polarization control device positioned on the incidence side and by positioning a polarization control device such as the polarizer which is in a crossed Nichols arrangement on the emitting side, and the selected light transmits through the polarization control device.
With this design, when voltage is applied to the liquid crystal layer, using ECB effects, the liquid crystal molecules tilt the molecule major axis direction thereof in the thickness direction of the liquid crystal layer, and the amount of birefringence in the liquid crystal layer thickness direction is lessened. Thus, the light wave having transited the liquid crystal layer changes to an elliptic polarization state according to voltage applied to the liquid crystal layer. The light components where the oscillating direction is not orthogonally transformed are interrupted, by the polarization control device positioned on the light emitting side. Thus, the device is configured so that the intensity of the incident light is modulated.
The basic operation principles of the liquid crystal modulators will be described using FIG. 5 and FIG. 6.
FIG. 5 is an operation description diagram of a case of using a transmissive liquid crystal modulator. In FIG. 5, the light from a light source (not shown) becomes linearly polarized light LIW via a polarization selector such as a polarizer not shown, and is cast into a transmissive liquid crystal modulator 300 with the polarized light from the arrow IW direction at a 45 degree angle with the orientation direction of the liquid crystal of the transmissive liquid crystal modulator 300.
In this event, the incident light LIW divides the liquid crystal layer of the transmissive liquid crystal modulators 300 into two characteristic modes and is propagated. The emitted light LOW is emitted in the direction of the arrow OW in the diagram, with the retardation δ(λ) shown in the following Expression (1) between the two characteristic modes.δ(λ)=2π(d·Δn)/λ  (1)
Here, λ is the wavelength of the incident light LIW, d is the thickness of the liquid crystal layer, and Δn is the refractive index anisotropy of the liquid crystal layer.
Next, the light LOW transits a polarization selecting device 301 such as a polarizer, which transmits linearly polarized light which is orthogonal to the polarization direction of the incident light LIW positioned on the emitting side. In this event, the transmissive liquid crystal modulators 300 are transmitted, and the amount of light transmitting the polarization selecting device 301, that is to say, the transmittance T(λ) of the transmissive liquid crystal modulators 300 are as follows.
If the transmittance of the polarization selecting device 301 is 100% as to the linearly polarized light to be transmitted, and the aperture ratio of the transmissive liquid crystal modulator 300 is 100%, and the non-polarized transmittance is 100%, then the transmittance T(λ) of the light LMW emitted in the MW arrow direction in the diagram which transits the polarization selecting device 301 as to the phase difference δ(λ) is expressed byT(λ)=0.5(1−cos(δ(λ))).  (2)
The transmittance of the liquid crystal modulators hereafter refers to the ratio of amount of light which transits the polarization selector 301 to the amount of light of the linearly polarized light cast into the liquid crystal modulators 300, via the polarization selectors as expressed in Expression (2).
When voltage is applied to the liquid crystal layer, the liquid crystal molecules move in the direction from parallel to perpendicular as to the sandwiched substrate of the liquid crystal layer, and thus the refractive index anisotropy Δn appears to be reduced. Therefore the retardation δ(λ) is reduced, and when δ=0 the transmittance T=0, and a black display is realized.
On the other hand, with no voltage applied, the refractive index anisotropy Δn is at its greatest, and if the liquid crystal layer thickness d and the refractive index anisotropy Δn of the liquid crystal layer is determined such that d·Δn=λ/2, then δ(λ)=π, the transmittance is T=1, and the display is brightest.
FIG. 6 is an operation description diagram using reflective liquid crystal modulators. In FIG. 6, the light LIW from the light source is cast into the polarizing beam splitter 401 from the IW arrow direction in the diagram, the light LIWB of the P components transit the polarizing selector film 401a in the IWB arrow direction in the diagram, and the light LIWA of the S components are reflected and deflected in the IWA arrow direction in the diagram. The light component LIWA of the arrow IWA includes the light selected which is linearly polarized in the vertical direction in the diagram.
The liquid crystal orientation direction of the reflective liquid crystal modulators 400 is tilted at a 45 degree angle as to the linearly polarized direction of the light LIWA. The light LIWA cast into the reflective liquid crystal modulators 400 from the IWA arrow direction divides the liquid crystal layer of the reflective liquid crystal modulators 400 into two characteristic modes that are propagated. Then when the light LOW is reflected and emitted in the direction of the arrow OW in the diagram. The light LOW is emitted with the retardation δ(λ) between the two modes, expressed in the following Expression (3).δ(λ)=2π(2d·Δn)/λ  (3)
Here, λ is the wavelength of the incident light, d is the thickness of the liquid crystal layer, and Δn is the refractive index anisotropy Δn of the liquid crystal layer.
Then, the light LOW emitted in the OW arrow direction in the diagram, the light LBW of the vertical direction component (S-polarization component as to the polarizing beam splitter 401) is reflected in the BW arrow direction in the diagram by the polarization separation plane 401a and returns to the light source side, and the light LMW of the parallel direction component (P-polarization component as to the polarizing beam splitter 401) is transmitted in the MW arrow direction in the diagram by the polarization separation plane 401a. The amount of light which reflects from the reflective liquid crystal modulators 400 and transmits through the polarizing beam splitter 401, that is to say, the reflectivity R(λ) of the reflective liquid crystal modulators 400 can be expressed as follows. If the S-polarizing reflectivity of the polarizing beam splitter 401 is 100%, the P-polarizing transmittance is 100%, and the aperture ratio of the reflective liquid crystal modulators 400 is 100% and the non-polarizing reflectivity is 100%, then the reflectivity (light transfer rate) R(λ) emitted in the MW arrow direction in the diagram as to the retardation δ(λ) is expressed asR(λ)=0.5(1−cos δ(λ))).  (4)
The reflectivity of the reflective liquid crystal modulators refers to the ratio of amount of light which transits the polarizing beam splitter 401 as to the amount of light of the linearly polarized light cast into the liquid crystal modulators 400, via the polarizing beam splitter 401a as expressed in Expression (4).
When voltage is applied to the liquid crystal layer, the liquid crystal molecules move in the direction from parallel to perpendicular as to the sandwiched substrate of the liquid crystal layer, and thus the refractive index anisotropy Δn appears to be reduced. Therefore the retardation δ(λ) is reduced, and when δ=0 the reflectivity R=0, and display becomes black.
On the other hand, with no voltage applied, the refractive index anisotropy Δn is at its greatest, and if the liquid crystal layer thickness d and the refractive index anisotropy Δn is determined so that 2d·Δn=λ/2, then δ(λ)=π, the reflectivity is R=1, and the display is clearest.
With the liquid crystal modulators which perform modulation control using the ECB effect control in this TN mode, there are restrictions regarding the light wavelength λ and amount of applied retardation which indicates the absolute amount of the length not dependent on the light wavelength of the retardation δ. According to the principles described above, it is apparent that the phase difference δ is an amount dependent on the wavelength. In other words, the wavelength band of the incident light of the three primary colors each have a wavelength band of R, G, B (red, green, blue), and the wavelength band has a width of slightly less than 100 nm.
Therefore, the TN-type liquid crystal modulators designed with a predetermined standard are configured so as to apply retardation of only a half-wavelength as to the predetermined wavelength light in a state with no voltage being applied to the liquid crystal layer. Thus, with the wavelength band of slightly less than 100 nm corresponding to each color, it is inevitable that retardation of greater than a half-wavelength can be applied, or retardation of less than a half-wavelength can be applied.
One more related restriction is that, because the TN mode is used, the birefringence of the liquid crystal layer is greatest when no voltage is being applied to the liquid crystal layer, and while the amount of retardation can be controlled in the direction of lessening by the ECB effect control, the opposite direction of increasing the amount of retardation is impossible.
On the other hand, a mainstream configuration is a full-color display type projection display device (color projector) which can use a liquid crystal modulator as a two-dimensional pixel optical switch which modulates the colored lights RGB (red, green, blue) which are the three primary colors of the additive color-mixing display as individual two-dimensional images, regardless of the transmissive liquid crystal modulators or reflective liquid crystal modulators, and after this, the full-color image is displayed using light-synthesizing means. This color project has a configuration of using three liquid crystal modulators, for modulating each of the R, G, and B light.
A color projector using such three liquid crystal display devices has been described in EP1447993A1.